1. Field of the Invention
The present invention relates to a laser driver for causing a laser to emit a laser beam, and more particularly to a laser driver for driving a vertical cavity surface emitting laser.
2. Description of the Related Art
A light emission resistance of a vertical cavity surface emitting laser is about one order of magnitude larger than that of an edge emitting laser and has a resistance value of about 100 Ω. In addition thereto, a parasitic capacitance of the vertical cavity surface emitting laser becomes large along with increased multiple channel. Moreover, the light emission resistance of the vertical cavity surface emitting laser changes due to a change in temperature following the light emission, and thus light emission characteristics also change.
In order to cope with such a situation, for the drive for the vertical cavity surface emitting laser, the light emission characteristics are stabilized by not using a voltage driving system with which the change in light emission resistance is influenced, but using a current driving system in many cases.
However, in the vertical cavity surface emitting laser, rising characteristics (TR characteristics) and fall characteristics (TF characteristics) which are not influenced so much in the case of the edge emitting laser depend on a time constant determined based on a resistance value of the light emission resistance, and a capacitance value of a parasitic capacitance parasitic in the vertical cavity surface emitting laser. To this end, the current driving system involves such a problem that a waveform of a drive signal for driving the vertical cavity surface emitting laser is driven is blunted in shape.
This problem will now be concretely described with reference to FIGS. 6A to 6D. FIG. 6B shows an equivalent circuit diagram of a laser L shown in FIG. 6A. Each of an edge emitting laser Lt and a vertical cavity surface emitting laser Lm is equivalently expressed in the form of a parallel circuit of a light emission resistance Ract and a parasitic capacitance Ca.
The edge emitting laser Lt has the small light emission resistance Ract and the small parasitic capacitance Ca. For example, a resistance value of the light emission resistance Ract is about several tens of Ω. On the other hand, the resistance value of the light emission resistance Ract of the vertical cavity surface emitting laser Lm is about one order of magnitude than that of the edge emitting laser Lt. In addition thereto, since a large number of vertical cavity surface emitting lasers Lm are formed on one plane, each of lengths of distributed electrodes becomes long, which results in that a capacitance is readily formed between each two adjacent electrodes.
As has been described, an area of a light-emitting face of the vertical cavity surface emitting laser Lm is larger than that of the edge emitting laser Lt, which results in that the light emission resistance Ract and parasitic capacitance Ca of the vertical cavity surface emitting laser Lm are each larger than those of the edge emitting laser Lt.
Here, FIG. 6D shows a waveform chart of a current IR which is caused to flow through the vertical cavity surface emitting laser Lm when the current IR is supplied from a current source 100 to the vertical cavity surface emitting laser Lm.
When the supply of the current IR from the current source 100 is started, almost the current IR supplied from the current source 100 is caused to flow through the parasitic capacitance Ca. Also, as the parasitic capacitance Ca is charged with the electric charges originating from the supplied current, the current supplied to the parasitic capacitance Ca is gradually reduced, and thus the current IR supplied to the light emission resistance Ract gradually increases. When the charging of the parasitic capacitance Ca with the electric charges is completed, all the current IR supplied from the current source 100 is caused to flow through the light emission resistance Ract. On the other hand, as soon as the current supply from the current source 100 is stopped, the current IR starts to be supplied from the parasitic capacitance Ca in which the electric charges are accumulated based on the charging to the light emission resistance Ract.
As a result, the waveform of the current IR supplied from the current source 100 to the vertical cavity surface emitting laser Lm is blunted at the time of the rising thereof, as well as at the time of the fall thereof in accordance with a time constant CaRact. This means that a modulation speed when a laser beam is modulated becomes slow.
In order to cope with this problem, in the related art, the rising and fall characteristics (hereinafter referred to as “the TR/TF characteristics”) of the vertical cavity surface emitting laser are improved by providing a differential current generating circuit.
FIG. 7A is a circuit diagram explaining a concept of an improvement in the TR/TF characteristics by providing a differential current generating circuit. In addition, FIG. 7B is a waveform chart of a current pulse supplied from a current source 110, and FIG. 7C is a waveform chart of a differential current. Also, FIGS. 7D and 7E are waveform charts of currents each supplied to the vertical cavity surface emitting laser.
A rectangular current pulse is supplied from the current source 110 to the vertical cavity surface emitting laser Lm to drive the vertical cavity surface emitting laser Lm (refer to FIG. 7B). On the other hand, a voltage V110 rises synchronously with the rising of the rectangular current pulse supplied from the current source 110 to supply a positive current differentiated through a capacitor C100 to the vertical cavity surface emitting laser Lm (refer to FIG. 7C). In the manner described above, the rising characteristics of the vertical cavity surface emitting laser Lm are improved (refer to FIG. 7D). In addition, the voltage V110 falls synchronously with the fall of the current pulse supplied from the current source 110 to supply a negative current differentiated through the capacitor C100 to the vertical cavity surface emitting laser Lm (refer to FIG. 7C). In the manner described above, the fall characteristics of the vertical cavity surface emitting laser Lm are improved (refer to FIG. 7D).
For example, with the techniques disclosed in Japanese Patent Laid-Open Nos. 2002-76504, 2008-113050 and 2008-113051, respectively, a complementary current source is provided in a drive source for a vertical cavity surface emitting laser, and a differential current is supplied from the complementary current source to the vertical cavity surface emitting laser, thereby improving the TR/TF characteristics of the vertical cavity surface emitting laser. In addition, it is proposed that a control voltage for the complementary current source is sampled and held, thereby controlling a voltage developed across both ends for application of the differential current, a complementary waveform is compared with a reference, thereby adjusting an amount of compensation, and a compensation capacitance is switched over to another one every vertical cavity surface emitting laser having multiple channels, thereby carrying out the driving operation.
In addition, a driving method of driving a vertical cavity surface emitting laser by switching a voltage driving driver, and a current driving driver over to each other is proposed in Japanese Patent Laid-Open No. 2008-98657 (refer to FIG. 15). The vertical cavity surface emitting laser emits a laser beam, so that a temperature of the vertical cavity surface emitting laser rises. A rise of the temperature of the vertical cavity surface emitting laser results in that a light emission resistance changes and thus an amount of light emission fluctuates. In order to prevent this situation from being caused, with the technique disclosed in Japanese Patent Laid-Open No. 2008-98657, after the vertical cavity surface emitting laser is driven for a given time period in accordance with the voltage drive, the voltage drive is switched over to current drive, thereby maintaining the amount of light emission.